Bow and Un-Bow

Monika Landy-Gyebnar of Hungary planned carefully in order to capture this panorama of raindrop refractions. At left the low sun has created a highly reddened primary rainbow. At right the sun shines through falling rain to give a wonderful red-golden light - the 'zero order glow' of a rainbow.

"I send you a rainbow & zero order glow panorama I made today [24th April '12] at sunset - just for its beauty.

The zero order glow was terribly vivid. The whole sky had a red/rosy tint but at the western horizon it looked like an A-bomb's light... Many of us had a rainbow today in Hungary and I received 9.5mm rain (most of it fell on my clothes and my hair). The beauty of this rainbow and the glow was in its timing. As a cold front's clouds travelled eastwards across our country I saw their end in the west two hours before sunset. I looked at the velocity of the clouds and said to myself: 'I have to prepare as I'm going to get that cloud gap just at sunset.' And 20 minutes before sunset the cloud gap appeared, the sun shone and made the rainbow and the glow. It was still raining but not too much.. it was simply perfect."

Image ©Monika Landy-Gyebnar
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The primary rainbow results from rays that enter raindrops, reflect from the opposite side and leave again deflected almost in the direction of the original sunlight. The two refractions separate the colours. We see a red bow when the sun is setting because scattering has denuded its light of greens and blues.

We only see a rainbow because the exit rays cluster at a minimum deviation or rainbow angle. Put another way, the exit rays form two zones (crossing and non-crossing) whose boundary is a bright caustic sheet.

Textbooks oft forget the rays passing through the drop without a reflection - the majority! With these there is no angle of minimum deviation. There are no two zones of different ray behaviour separated by a sharp and bright caustic. There is no rainbow. Instead there is a lovely diffuse golden glow where millions of raindrops between the eye and sun echo its light - a "zero order glow".

Look for rainbows opposite the sun - then look sunwards. Whenever the sky shows some optical effect be cantankerous and look in the opposite direction, often there is a surprise.